JSON Schema for the OKF YAML MD Frontmatter

How about writing a [JSON Schema](https://json-schema.org) for the [OKF YAML MD Frontmatter](https://github.com/GoogleCloudPlatform/open-knowledge-format/blob/main/SPEC.md)?

I would then use this e.g. to validate my https://github.com/enola-dev/wiki.

Maybe I'll write one, and contribute it here.

Unless someone gets to it before I do.

Start by reading SPEC.md to identify every field and constraint in the OKF YAML MD frontmatter. Then locate the appropriate place in the repository for a JSON Schema and determine how it can be checked against the format; done means the schema represents the documented frontmatter and can support validation of an external OKF wiki.

Written by the indexing model from the issue text.